A widely used monochromator for analyzing machines includes: a light dispersing element, such as a diffraction grating, for dispersing light with respect to the wavelength; a driving mechanism for rotating the light dispersing element; a slit for extracting a monochromatic light from the dispersed light; and a light detector for detecting the monochromatic light extracted by the slit and for generating a signal corresponding to the strength of the detected monochromatic light.
In a spectrophotometer using such a monochromator, two analyzing methods are generally available: a wavelength scanning analysis and a time course analysis. While, in the wavelength scanning analysis, the light dispersing element is rotated within a preset angle, the signal from the light detector is sampled and a spectrum of the light within a preset wavelength range (wavelength scanning range) is obtained. Such a wavelength scanning is repeated several times if necessary. In the time course analysis, on the other hand, the light dispersing element is fixed at a certain angular position corresponding to an object wavelength and the change in the strength of the light of the object wavelength with respect to time is measured.
Several types are practically used for the driving mechanism: one using a sine bar mechanism to convert a translational movement into a rotational movement; one using an open loop control with a stepping motor and a reduction gears; one using a closed loop control with a DC (direct current) servo motor, etc. When a diffraction grating is used for the light dispersing element, its rotating range is generally set at 20-30 degrees.
Among the above described three types of the driving mechanisms, the one using a sine bar and the one using a stepping motor have a common drawback that the wavelength scanning requires a long time. Sometimes it takes several tens of seconds for completing a round of wavelength scanning. Further, since in these mechanisms the rotational position of the light dispersing element is not detected, a discrepancy can easily occur between the wavelength of the actually obtained monochromatic light and the object wavelength if the relationship between the control value of the motor and the actual angular position of the motor is once distorted due to, for example, the degenerative change of the driving mechanism with respect to time.
In the DC servo motor mechanism, high speed wavelength scanning is possible and no such discrepancy occurs because the rotational position of the light dispersing element is always detected. But a normally available DC servo motor (or motor of similar kinds) is designed to be rotated continuously (i.e., rotated more than 360 degrees). When such a motor is used to rotate a light dispersing element within a small angular range (i.e., 20-30 degrees as described above), it is difficult to stabilize the rotational speed of the light dispersing element in such a small angular range. In addition to that, the rotational speed is not correctly reproduced if the motor is rotated back and forth repeatedly in such a small angular range.
The inventor of this application has proposed a new monochromator in the Japanese Patent Application No. H10-124167 (Publication No. H11-304587) for solving the above problems, in which the light dispersing element is driven by a mechanism including a liner motor and a position detector (encoder). Owing to the driving mechanism, the efficiency and accuracy of the wavelength scanning analysis of the monochromator is greatly improved. The new driving mechanism has a further advantage that its cost is low compared to conventional mechanisms.